1. Field of the Invention
The present invention relates to a diffraction spatial light modulator and to a display incorporating such a modulator. Such a display may be of the projection type and may be used to provide large screen TV viewing and business presentations.
2. Description of the Related Art
A known type of projection display is disclosed in a paper entitled xe2x80x9cHigh-Definition Projection System Using DMD Display Technologyxe2x80x9d, by G. Saxtro, T. Ballew, and J. Iwai, SID 1995, pages 70 to 73. This display uses digital micromirror devices (DMD) in which electrostatic forces are used to tilt planar micromirrors so as to selectively deflect light from spatial light modulator picture elements to a projection optical system. The individual micromirrors are attached to a CMOS chip by means of micro-fabricated pivot structures. A disadvantage of this type of display is that it is expensive to manufacture. Another disadvantage is that, during operation, the micromirrors have to make many rapid movements. This stresses the pivot structures, giving rise to reliability and working life issues.
Another type of projection display is disclosed in a paper entitled xe2x80x9cDeformable Grating Light Valves for High-Resolution Displaysxe2x80x9d by R. B. Apte, F. S. A. Sandejas, W. C. Banyai, and D. M. Bloom, SID 1993 pages 807 and 808. This system uses a micromechanical light valve with electrostatic control of a reflection phase grating. The grating comprises mirrored air bridge beams which are arranged to lie with the mirrored surfaces at a half wave above a second plane mirror. The air gap is a quarter wave which reduces to zero when an electrostatic field is applied. In the absence of a field, the structure acts as a mirror whereas, in the presence of a field, the structure acts as a diffraction grating. A similar type of arrangement is disclosed in U.S. Pat. No. 5,311,360.
Although this type of display is less expensive then the DMD type, it is of a similar type relying on a mechanical movement giving rise to reliability and working life issues. Further, this type of display has a poorer contrast performance and, because of the limited aperture ratio which can be achieved, provides lower brightness for a given light source.
Another type of projection display is disclosed in a paper entitled xe2x80x9cRecent Advances in Actuated Mirror Array (AMA) Projector Developmentxe2x80x9d by G. Um, D. Foley, A. Szilagyi, J. B. Ji, Y. B. Joen, and Y. K. Kim, Asia Display 1995, pages 95 to 98. This display is similar to the digital mirror device described hereinbefore but uses piezo driven mirrors instead of electrostatically driven mirrors. Schlieren bars are used to introduce light into the optical system. These are reimaged onto themselves when the picture elements are switched off. This type of display requires relatively high voltages for the piezo drivers and is therefore inconvenient. Further, it is difficult to fabricate and so is of lower commercial viability.
Another projection display is disclosed in a paper entitled xe2x80x9cCompact Liquid Crystal Projectors with High Optical Efficiencyxe2x80x9d by Y. Nagae, K. Ando, A. Asano, I. Takemoto, J. Havens, P. Jones, D. Reddy, and A. Tomita, SID 1995, pages 223 to 236. This display uses the nematic curvilinear aligned phase technique for selectively scattering light together with Schlieren optics. In particular, each pixel switches between a non-scattering mode, such as a reflective mode, in which light is directed into a well-defined path, and a scattering mode in which light is scattered over a range of angles. The display may therefore provide a good contrast ratio or high brightness but, because of its mode of operation, cannot simultaneously provide both a good contrast ratio and high brightness.
U.S. Pat. No. 5,434,690 discloses a liquid crystal device with pixel electrodes in an opposed striped form. When a field is applied to the striped electrodes of a pixel, light is scattered from the pixel. This type of device is only suitable for forming gratings which provide relatively small deflection angles of incident light. In order to provide acceptable efficiency, this requires the use of smaller more collimated light sources which in turn leads to unacceptable cost with poor reliability and a relatively short working life.
A spatial light modulator using ferroelectric liquid crystal technology is disclosed in a paper entitled xe2x80x9cDiffractive Ferroelectric Liquid Crystal Shutters for Unpolarised Lightxe2x80x9d by M. J. O""Callaghan and M. A. Handschy, Optics Letters, Volume 16 No. 10, May 1995, pages 770 to 772 and in U.S. Pat. No. 5,552,916. The spatial light modulator disclosed in this paper is switchable between a first state in which it transmits incident light and a second state in which it acts as a phase diffraction grating. However, this type of modulator relies on the use of ferroelectric liquid crystals which are capable of providing a 90xc2x0 switching angle of their optic axis. Such materials are available but exhibit very poor performance. For instance, the switching rate is slow and is inadequate for use at normal video refresh rates. Also these materials cannot be aligned adequately to avoid scattering of light.
Another spatial light modulator is disclosed in a paper entitled xe2x80x9cImproved Transmission in a Two-Level, Phase Only, Spatial Light Modulatorxe2x80x9d by M. A. A. Neal and E. G. S. Page Electron. Lett. 30 (5) pages 445-6 1994. This paper discloses a spatial light modulator which is switchable between a non-diffractive mode and a diffractive mode in which alternative strips of the modulator rotate unpolarized light by plus and minus 45 degrees and an associated half wave retarder further rotates all the polarisation components of the light so as to provide phase-only modulation. This paper is exclusively concerned with providing efficient diffraction of polarised and wall-collimated light from a laser source. The modulator, the half wave retarder and a mirror are embodied as physically separate spaced elements or devices. Such an arrangement is therefore unsuitable for use as a display.
EP 0 811 872 discloses a diffractive spatial light modulator of the same type in which the various retarders (including one or more liquid crystal layers) and, in the case of reflective devices, the mirror are disposed between the substrates. Such devices may be used in projection displays.
U.S. Pat. No. 5,182,665 discloses a similar type of spatial light modulator.
According to a first aspect of the invention, there is provided a diffractive spatial light modulator comprising first and second substrates, a half wave retarder disposed between the first and second substrates, a quarter wave retarder disposed between the half wave retarder and the second substrate, and a reflector disposed between the quarter wave retarder and the second substrate, the half wave retarder comprising a plurality of picture elements, each of which comprises a plurality of first elongate regions interdigitated with a plurality of second elongated regions, the first regions having optic axes which are switchable between a first direction and a third direction, the second regions having optic axes which are switchable between a second direction and the third direction, and the quarter wave retarder having a fixed optic axis, the first and second directions being oriented at angles xcex11 and xcex12, respectively, relative to an arbitrary reference direction and the optic axis of the quarter wave retarder being oriented at an angle xcex2 relative to the reference direction such that (xcex11-xcex12) less than 90xc2x0 and xcex12+180xc2x0 greater than xcex2 greater than xcex11.
(xcex11-xcex12) is preferably between 35xc2x0 and 55xc2x0, more preferably between 40xc2x0 and 50xc2x0 and most preferably substantially equal to 45xc2x0.
The optic axis of the quarter wave retarder may be substantially perpendicular to the bisector of the first and second directions.
The half wave retarder may comprise a liquid crystal layer. The liquid crystal layer may comprise a ferroelectric liquid crystal layer. The liquid crystal layer may be disposed between first and second alignment layers. Each of the first and second alignment layers may comprises an obliquely evaporated layer of silicon oxide.
The first transparent electrode layer may be disposed between the first substrate and the half wave retarder. A second electrode layer may be disposed between the second substrate and the half wave retarder. At least one of the first and second electrode layers may comprise a plurality of electrode picture elements aligned with the picture elements of the half wave retarder, each electrode picture element comprising first and second electrode regions aligned with the first and second regions, respectively, of the half wave retarder. The second electrode layer may constitute the reflector.
According to a second aspect of the invention, there is provided a diffractive spatial light modulator characterised by first and second substrates, a first half wave retarder disposed between the first and second substrates, a second half wave retarder disposed between the first half wave retarder and the second substrate, a third half wave retarder disposed between the second half wave retarder and the second substrate, the first retarder comprising a plurality of picture elements, the third retarder comprising a plurality of picture elements aligned with the picture elements of the first retarder, each of the picture elements of each of the first and third retarders comprising a plurality of first elongate regions interdigitated with a plurality of second elongate regions, the first regions having optic axes which are switchable together between a first direction and a third direction, the second regions having optic axes which are switchable together between a second direction and the third direction, and the second half wave retarder having a fixed optic axis, the first and second directions being oriented at angles xcex11 and xcex12, respectively, relative to an arbitrary reference direction and the optic axis of the second half wave retarder being oriented at an angle xcex2 relative to the reference direction such that (xcex11-xcex12) greater than 90xc2x0 and xcex12+180xc2x0 greater than xcex2 greater than xcex11.
(xcex11-xcex12) is preferably between 35xc2x0 and 55xc2x0, more preferably between 40xc2x0 and 50xc2x0 and most preferably substantially equal to 45xc2x0.
The optic axis of the second half wave retarder may be substantially perpendicular to the bisector of the first and second directions.
Each of the first and third retarders may comprise a liquid crystal layer. Each of the liquid crystal layers may comprise a ferroelectric liquid crystal layer. Each of the liquid crystal layers may be disposed between first and second alignment layers. Each of the first and second alignment layers may comprise an obliquely evaporated layer of silicon oxide.
A first transparent electrode layer may be disposed between the first substrate and the first half wave retarder. A second transparent electrode layer may be disposed between the second substrate and the third retarder. At least one of the first and second electrodes may comprise a plurality of electrode picture elements aligned with the picture elements of the first and third retarders, each of the electrode picture elements comprising first and second electrode regions aligned with the first and second regions, respectively, of the first and third retarders.
According to a third aspect of the invention, there is provided a display characterised by a spatial light modulator according to the first or second aspect of the invention.
The display may comprise a light source for illuminating the modulator and an optical projection system for receiving light only from a predetermined diffractive order from the modulator. The predetermined diffractive order may be the zeroth diffractive order. As an alternative, the predetermined diffractive order may be a first diffractive order.
It is thus possible to provide a spatial light modulator which overcomes the disadvantages of the prior art as described hereinbefore. Such a modulator can have individual high resolution picture elements which are switchable between a non-diffractive mode and a diffractive mode for use with unpolarised light. A display using such a spatial light modulator has good reliability and long operational life while providing both high intensity and high contrast ratios.